Polymers are attractive materials for various applications, such as membrane separation, coatings, composites, microelectronic devices, thin-film technology, biomaterials, and so on. The performance of polymeric materials in many applications relies largely upon the combination of bulk (e.g. mechanical) properties in combination with the properties of their surfaces. However, polymers very often do not possess the surface properties needed for these applications. Vice versa, those polymers that have good surface properties frequently do not possess the mechanical properties that are critical for their successful application. Due to this dilemma, (surface) modification of polymers without changing the bulk properties has been a topical aim in research for many years, mostly because surface modification provides a potentially easier route than e.g. polymer blending to obtain new polymer properties.
In membrane separation processes, membrane fouling is a serious problem that may be reduced (or even prevented) using surface modification. Membrane fouling is the accumulation of substances on the membrane surface and/or within the membrane pores, which results in deterioration of membrane performance. The interaction between membrane surfaces and solution components plays an important role in the extent of membrane fouling. In ultrafiltration of e.g. protein-containing liquids, fouling occurs due to protein adsorption, denaturation, and aggregation at the membrane solution interface.
Aromatic polysulfones, also known as poly(arylsulfones), form a well known class of thermoplastic polymers (see for example Kirk-Othmer, “Encyclopedia of Chemical Technology”, John Wiley & Sons, 4th Ed. 1996, Volume 19, p. 945-968). Poly(arylsulfones) are generally characterized by high glass-transition temperatures, good mechanical strength and stiffness, and excellent thermal and oxidative resistance. The backbone of a poly(arylsulfone) comprises sulfone, aryl and ether moieties as basic repeat units. Furthermore, additional connecting units may be present, such as for example isopropylene groups in polysulfone (PSF). Examples of poly(arylsulfones) include polysulfone (PSF), polyethersulfone (PES) and polyphenylsulfone (PPSF), and the repeat units of these polymers are shown below.

Poly(arylsulfones) may be prepared via a nucleophilic substitution polycondensation route, wherein 4,4′-dichlorodiphenylsulfone is reacted with a bisphenol of choice (the phenol group is the nucleophile) in the presence of a base. By using a different bisphenol in the polycondensation reaction, a different type of poly(arylsulfone) with a different repeating unit and different bulk properties may be obtained. Bisphenols that may be used in the polycondensation reaction with dichlorodiphenylsulfone are for example 4,4′-dihydroxydiphenyloxide, 4,4′-dihydroxydiphenylsulfide, 4,4′-dihydroxydiphenylmethane, 2,2-bis(4-hydroxyphenyl)-propane (for the preparation of PSF), hydroquinone, 2,2-bis(4-hydroxyphenyl)-perfluoropropane, 4,4′-dihydroxybenzophenone, 4,4′-dihydroxydiphenylsulfone (for the preparation of PES), 4,4′-dihydroxydiphenyl (for the preparation of PPSF), 1,4-bis(4-hydroxyphenyl)benzene or 4,4′-bis(4″-hydroxybenzenesulfonyl)diphenyl.
Poly(arylsulfones) are very suitable polymers for the manufacture of membranes such as for example ultrafiltration and microfiltration membranes. Due to their structural and chemical stability, poly(arylsulfones) provide robustness to the membranes. However, poly(arylsulfones) have a hydrophobic nature, with a relatively low surface energy and high water contact angle, and membranes made from these materials are therefore vulnerable to adsorptive fouling. This disadvantage may be reduced by decreasing the hydrophobicity of poly(arylsulfones), for example via (surface) modification of the polymer. Various methods for the surface modification of poly(arylsulfone) and poly(arylsulfone) membranes are described in the prior art, such as for example coating, blending, composite, chemical, and grafting methods, and combinations thereof.
Blending is a process in which two (or more) polymers are physically mixed to obtain the required properties. Blend polymer membranes based on polyethersulfone in combination with, amongst others, polyethylene glycol, poly(vinylpyrrolidone) or cellulose acetate are known in the art.
Coating is a method wherein the coating material forms a thin layer that non-covalently adheres to the substrate. An example of the coating of polysulfone membranes with methyl methacrylate-based comb polymers with short oligoethylene glycol side chains that provide the membrane with long-term, bio-repellent surfaces is disclosed by Hyun et al. (J. Membr. Sci., 2006, 282, 52-59, incorporated by reference). Cell-lysate flux recovery increased from 47% for unmodified polysulfone membranes to 94% for the coated membrane, and presumably this is caused by the hydrophilic polyethylene oxide groups on the surface.
For chemical modification, the membrane material is treated with modifying agents to introduce various functional groups on the membrane surface. For example WO 2009/024973, incorporated by reference, discloses a chemically modified polysulfone polymer substituted in one or more of the phenyl rings by functional groups selected from (i) —CO—R1, wherein R1 is OH, halohydrocarbyloxy, a mono- or oligosaccharide residue or a derivative thereof, (ii) —CON(R2)R3, wherein R2 is H or hydrocarbyl and R3 is a monosaccharide or oligosaccharide residue or a derivative thereof, (iii) —B(OR2)2 wherein R2 is H or hydrocarbyl, (iv) —P(═O)(OR2)2 wherein R2 is H or hydrocarbyl and (v) —CO—O—R4—O—CO— linking two chains of the polymer backbone, wherein R4 is alkylene. These modified polysulfones are suitable for composing membranes.
Grafting is a method wherein monomers, oligomers or polymers are covalently bonded onto the membrane. Grafting may occur either through ‘grafting-to’ polymerization (coupling of polymers to the surface of the membrane), or through ‘grafting-from’ polymerization (monomers are polymerized using an initiation site on the membrane surface). Polysulfone-graft-copolymers are disclosed in for example WO 2009/098161, incorporated by reference. WO 2009/098161 relates to alkoxyamine functionalized polysulfones, to a process for the functionalization of polysulfones with nitroxide initiators and subsequent nitroxide-mediated radical polymerizations to yield polysulfone-graft-copolymers, and to these graft-copolymers, which may be used as membranes.
Chemical surface modification and surface grafting methods result in a covalent attachment of the substituents and/or graft chains, and have the advantage of a long-term chemical stability. This stability is not obtained with for example physically coated polymer chains that may often be removed rather easily. In addition, grafting and chemical modification methods have the advantage that modification of the polymer surface to have distinct properties is feasible through the choice of different substituents.
One of the disadvantages of the chemical and grafting modification methods known in the art is that, due to the inert nature of poly(arylsulfones), severe reaction conditions are necessary for the modification, running the risk of undesirable surface changes, undesired side-reactions and degradation reactions, and contamination.